Police said Samir Ben Abdellah, a 36-year-old Moroccan who once worked as an imam in a Madrid suburb, was arrested near Barcelona on Monday on the orders of anti-terror judge Baltasar Garzon as part of an ongoing investigation into radical Islamic groups.

Police only made the arrest public late Tuesday.

Ben Abdellah, who will be transferred to Madrid on Wednesday, is thought to have links to Mohamed Achraf, the suspected ringleader of a plot to blow up Spain's supreme court in the capital and believed to be behind a radical group dubbed "martyrs for Morocco".

Spain is seeking the extradition of Achraf from Switzerland, where he is currently in custody.

At least three of 38 people Spain has arrested in recent weeks are believed linked to Allekema Lamari, one of seven suspects involved in planning the Madrid train bombings which killed 191 people on 11 March who blew themselves up during a police raid on a flat in a Madrid suburb in April.